I tend to view these type of things as "why CAN'T you add it?" Unless you're mixing ammonia and bleach, it's usually a pretty safe experiment.

edited to add: Look at it this way ... if your options are "can I mix it?" or "Do I make a new batch?" then go ahead and mix it. If it doesn't work, you just have to make a new batch, which was the other option anyway.

edited to add: Look at it this way ... if your options are "can I mix it?" or "Do I make a new batch?" then go ahead and mix it. If it doesn't work, you just have to make a new batch, which was the other option anyway.